How to mix to material with mix node
a node setup is a group of nodes that you build, to get a virtual material for a single object. As i see in your images, there are 2 objects, a "landscape"/mountain and a plane. You need to do that material just to the mountain (maybe extend the mountain mesh, or joining the meshes). On your node setup, under "texture coordinate", try to use "object" instead of "generated". To control the transition, in the color ramp node, change the mode until you get want you like.

On your node setup, connect directly the mix shader to the material output. Delete the diffuse shader.

The ocean should be just a plane that cuts the landscape and you move on Z axis, up and down to where you want it.



For a 3 material setup, procede as the following image, where you replace the RGB nodes with the materials you built: